The manufacture of semiconductor devices such as solar cells and computer chips is conducted through processes such as metalorganic chemical vapor deposition (MOCVD) wherein volatile sources of arsenic are useful as reactants therein. Such sources of arsenic must further be of high quality for use in such applications. Arsine has typically served as the volatile arsenic source. Use of arsine is undesirable because it is difficult to purify and especially because it is used as a compressed gas under elevated pressure, with resulting safety and environmental hazards due to the high toxicity of arsine and its persistence in the atmosphere. In recent years, alkylarsines, and especially mono-tertiary-butylarsine, have gained acceptance as replacements for arsine, due to their lower toxicity and presence in the liquid state under normal storage and handling conditions. Mono-arsines for use in semiconductor applications must be of high chemical purity, containing minimum amounts of stereoisomers or other chemical compounds and, especially, substantially no metallic or oxygenating impurities. Use of such very high purity mono-alkylarsines makes possible the fabrication of high quality semiconductor materials with properties equal or superior to materials made using arsine gas, but without the health and environmental hazards associated with use thereof. Therefore, a process is needed to make very high purity mono-arsines, containing virtually no metallic or oxygenating impurities.
One method to produce alkylarsines involves a Grignard reaction with arsenic trichloride to produce an alkylated arsenic chloride derivative, which is converted to the alkylated arsenic hydride (alkylarsine) by treatment with LiAlH.sub.4, NaBH.sub.4, Zn/HCl, or the like. Mono-tertiary-butylarsine was prepared by Tzschach et al. Z. anorg. allgem. Chem. 336, 36 (1965), using the above method. It has been found that mono-tertiary-butylarsine prepared by the above method contains characteristic germanium and other donor impurities, derived from the many reagents and solvents used, which are incompatible with growth of high quality semiconductors.
An alternative route to alkylarsines, disclosed for example in U.S. Pat. No. 5,068,372, is the reaction of an alkyl iodide and sodium arsenite to give an arsenious acid, (alkyl)As(OH).sub.2, which can be converted to the corresponding (alkyl)AsH.sub.2 by treatment with LiAlH.sub.4, NaBH.sub.4, Zn/HCl, or the like. This method is, however, limited to the use of primary and a few secondary alkyl iodides, and cannot be used to prepare mono-tertiary-butylarsine.
Alternatively, the appropriate olefin may be reacted with the arsine in the liquid phase containing a liquid (U.S. Pat. No. 4,857,655) or solid acid catalyst (U.S. Pat. No. 5,003,093). These processes can be used to make high purity mono-tertiary-butylarsine. However, because of facile formation of di-tertiary-butylarsine in condensed phase reactions, it is necessary to use high arsine pressures (400 psi) and excess arsine, which it is difficult to recover and recycle. Even when high arsine pressures are used, the highest obtained mono-/di-tertiary-butyl arsine ratios are only approx. 3/1. Formation of the unwanted di-tertiary-butylarsine lowers yields of the desired mono-tertiary-butylarsine. Acid catalyzed formation of mono-tertiary-butylarsine in condensed phase thus involves extreme safety hazards due to use of compressed arsine, and severe disposal problems due to formation of other organoarsine products in addition to mono-tertiary-butylarsine and to the need to dispose of large quantities of aqueous acid wastes containing arsenated species.
It is therefore an object of the present invention to provide a process which is highly selective for the desired alkylarsine, and in particular, for mono-tertiary-butylarsine, and produces a high yield of product of relatively high quality, thereby obviating the necessity of costly purification procedures. It is a further object of the present invention to provide a process which does not require high reaction (arsine) pressures. It is another object of the invention to provide a process which does not require disposal of significant quantities of reactants and/or byproducts. It is still another object of the present invention to provide a process for the production of alkyl arsines which may be run in a continuous fashion.